The most important filter material used worldwide for purifying and filtrating tobacco smoke, is cellulose acetate that may or may not be used in combination with activated carbon.
The usual filter materials, such as cellulose acetate and activated carbon, have the drawback of not removing a considerable part of the toxic compounds, such as tar and PAHs, present in tobacco smoke.
Furthermore a cigarette filter is described in British patent specification GB 755.057, which filter is formed from a urea formaldehyde foam having a fine foam structure containing cavities that are connected to each other so that tobacco smoke can flow through them, a filtration effect arising with respect to tar and nicotine present in the smoke.
The foam from GB 755.057 is prepared by first foaming a foamable liquid by injecting air into it or by stirring it. Subsequently the foamed liquid is transformed into a durable foam by admixing urea formaldehyde and a suitable catalyst with the liquid. After that the prepared foam is treated with an excess of an aqueous urea solution, in order to thus remove the smell of formalin.
Subsequently the foam treated with the urea solution is subjected to a compression process in order to increase the mechanical stability of the foam and at the same time to ensure that at least some of the hollow spaces remain intercommunicating. In this way, according to GB 755.057, a foam material having a skeleton-like fine structure and intricate shape is produced, comprising a plurality of flow ducts, a relatively low flow resistance and a substantially improved mechanical stability. Said fine structure filtrates the aerosol particles that are carried along by the smoke.
A drawback of the method of preparation according to GB 755.057 is that in the first step, that is to say the foaming of a foamable liquid, mixing takes place under ill-controllable conditions. As a result it is not possible to form a foam of a well-defined structure under well-controlled conditions.
Indeed the structure of the foam is of importance to the filtration effect with regard to tar and nicotine from tobacco smoke. It appeared to applicant that the structure of the foam prepared with the method according to GB 755.057 is not sufficiently effective to filtrate tar and nicotine from tobacco smoke. For instance the aforementioned structure does indeed comprise cavities, however including closed cavities which are not accessible to the aerosol particles carried along by the smoke.
Another drawback of the method of preparation according to GB 755.057 is that after preparation of the urea formaldehyde-based foam, said foam has yet to be subjected to a compression process in order to obtain a well-filtrating foam for a cigarette filter, by breaking the walls of the cavities.
Furthermore, in the Italian patent specification IT 574.416 a method for preparing a urea resin filter mass for the removal of nicotine and tar and other products from tobacco smoke is described. In said method a mixture of a urea formaldehyde solution, a foaming agent, an acid and finely powdered SiO2 gel is stirred until a thick foam forms, which is then allowed to solidify and is dried. SiO2 gel is a strong nicotine-bonding agent.
A drawback of the method according to IT 574.416 is that a SiO2 gel has to be used to obtain a sufficient filtration effect for the final foam product. Another drawback is that mixing takes place under ill-controllable conditions so that, just like in the method of preparation according to GB 755.057, it is not possible to form a foam of well-defined structure.